Phonon deformation potentials in hexagonal GaN

In the first part of this paper, we review the deformation potentials of the E-2 and A(1)(LO) phonons of hexagonal GaN proposed seven years ago, from a Raman study of stained layers. Elastic constants recently published are taken into account, and the optical calibration of strain is corrected. We show that new values of the biaxial pressure coefficients are only 10% higher than their previous determinations, leading to 3.2 cm(-1)/GPa and to 0.9 cm(-1)/GPa, for the E-2 and the A(1)(LO) modes, respectively. In the second part, we present another experimental determination of deformation potentials for these phonons. For the present study, a set of undoped epitaxial GaN layers grown on silicon by molecular beam epitaxy has been investigated by means of Raman scattering and x-ray diffraction. Values deduced from these measurements are compared to existing data. On the one hand, the Raman biaxial pressure coefficient has been found as low as 2.43 cm(-1)/GPa for the E-2 phonon while its counterpart for the A(1)(LO) phonon (1.91 cm(-1)/GPa) is much higher than its previous determination; this value will be of strong importance to calibrate strains in GaN-based nanostructures.